Following subcutaneous injection, radiolabeled monoclonal antibodies bind efficiently to normal and tumor target cells in the lymph nodes. This finding prompted us to attempt specific therapy using monoclonal antibodies covalently coupled to plant toxins. The first development along these lines has been to synthesize monoclonal antibody-ricin A-chain conjugates using four antibodies of different specificities. We then demonstrated the capacity of these conjugates to bind to their target cells and to inhibit protein synthesis at the ribosomal level in an acellular system. Unfortunately, using the guinea pig hepatocarcinoma cell line (L 10), which expresses large quantities of target antigen, we found only weak